FIELD OF THE INVENTION
The invention relates to a switchable current source circuit including an electronic switch; and a current mirror configuration having a control stage furnishing a control voltage at a voltage output as a function of a control current impressed at a current input, and an output stage having a control input connected to the voltage output of the control stage and a control path with two terminals, one of which forms a current output of the current source circuit and the other of which is connected to one terminal of the electronic switch, the other terminal of the electronic switch being connected to a supply potential terminal, and the current source circuit being switchable on and off as a function of the triggering of the control input of the electronic switch. The invention also relates to a phase detector configuration having such a circuit.
In particular, the Siemens data book entitled "ICs for Radio Equipment 1989/90", pp.51-72, describes the TBB 206 integrated circuit, which is commercially available, and its use in a PLL. The phase detector used in that circuit has an output stage including two current source circuits, which furnish currents with different signs to a common circuit node forming the output. The analog-controlled current sources used in that case do not reach their desired output current until a relatively long time after being triggered, which is approximately 30 ns. Such a long rise time of the output currents of those current sources also makes the width of the anti-backlash pulses that are required in such applications relatively great. Due to those anti-backlash pulses, the output spectrum of the phase detector includes not only the desired output frequency but also a large number of secondary lines, each being spaced apart from an input reference frequency of the phase detector. If those anti-backlash pulses are relatively wide, then the power content of the secondary lines is relatively high, so that an application in a phase-locked loop requires the use of an expensive filter, in order to adhere to a specified ratio between the signal level and the noise level.
Current mirror circuits per se are described, among other sources, in the book entitled "Halbleiterschaltungstechnik" [Semiconductor Circuitry] by Tietze and Schenk, 9th Edition, 1991, in both chapter 4.6.3, pp. 62-64, particularly FIG. 4.36, and chapter 5.5, pp. 94-97, particularly FIG. 5.13. Such current mirror circuits have a control stage on which a control current is impressed and which furnishes a control voltage as a function of the impressed control current. Moreover, such current mirror circuits have an output stage, which furnishes an output current as a function of the control voltage of the control stage. The term "current mirror circuit" is used herein to mean that the output current is proportional to the input current. Especially in integrated circuits, a current transfer is brought about with the aid of a transistor area ratio, in which the effective area of a transistor forming the output stage is greater by a certain factor than the effective area of a transistor forming the control stage.
It is also known that the output stage, which is typically hard-wired to a reference potential, is switchable to that reference potential through an electronic switch. If that electronic switch is made conducting, the constant current then flows at the current output of that current source, while if that electronic switch is blocked, no current flows at the current output of the current source. When such switchable current sources are turned on, the control voltage furnished by the control stage drops somewhat at the turn-on time, so the desired constant current at the current source circuit output does not flow immediately.